287 research outputs found
Tight-binding molecular-dynamics studies of defects and disorder in covalently-bonded materials
Tight-binding (TB) molecular dynamics (MD) has emerged as a powerful method
for investigating the atomic-scale structure of materials --- in particular the
interplay between structural and electronic properties --- bridging the gap
between empirical methods which, while fast and efficient, lack
transferability, and ab initio approaches which, because of excessive
computational workload, suffer from limitations in size and run times. In this
short review article, we examine several recent applications of TBMD in the
area of defects in covalently-bonded semiconductors and the amorphous phases of
these materials.Comment: Invited review article for Comput. Mater. Sci. (38 pages incl. 18
fig.
Identification of relaxation and diffusion mechanisms in amorphous silicon
The dynamics of amorphous silicon at low temperatures can be characterized by
a sequence of discrete activated events, through which the topological network
is locally reorganized. Using the activation-relaxation technique, we create
more than 8000 events, providing an extensive database of relaxation and
diffusion mechanisms. The generic properties of these events - size, number of
atoms involved, activation energy, etc. - are discussed and found to be
compatible with experimental data. We introduce a complete and unique
classification of defects based on their topological properties and apply it to
study of events involving only four-fold coordinated atoms. For these events,
we identify and present in detail three dominant mechanisms.Comment: 4 pages, three figures, submitted to PR
Amorphous silicon under mechanical shear deformations: shear velocity and temperature effects
Mechanical shear deformations lead, in some cases, to effects similar to
those resulting from ion irradiation. Here we characterize the effects of shear
velocity and temperature on amorphous silicon (\aSi) modelled using classical
molecular dynamics simulations based on the empirical Environment Dependent
Inter-atomic Potential (EDIP). With increasing shear velocity at low
temperature, we find a systematic increase in the internal strain leading to
the rapid appearance of structural defects (5-fold coordinated atoms). The
impacts of externally applied strain can be almost fully compensated by
increasing the temperature, allowing the system to respond more rapidly to the
deformation. In particular, we find opposite power-law relations between the
temperature and the shear velocity and the deformation energy. The spatial
distribution of defects is also found to strongly depend on temperature and
strain velocity. For low temperature or high shear velocity, defects are
concentrated in a few atomic layers near the center of the cell while, with
increasing temperature or decreasing shear velocity, they spread slowly
throughout the full simulation cell. This complex behavior can be related to
the structure of the energy landscape and the existence of a continuous
energy-barrier distribution.Comment: 10 pages, 17 figure
Mitigating Alzheimer’s disease with natural polyphenols: a review
According to Alzheimer’s Disease International (ADI), nearly 50 million people worldwide were living with dementia in 2017, and this number is expected to triple by 2050. Despite years of research in this field, the root cause and mechanisms responsible for Alzheimer’s disease (AD) have not been fully elucidated yet. Moreover, promising preclinical results have repeatedly failed to translate into patient treatments. Until now, none of the molecules targeting AD has successfully passed the Phase III trial. Although natural molecules have been extensively studied, they normally require high concentrations to be effective; alternately, they are too large to cross the blood-brain barrier (BBB). In this review, we report on AD treatment strategies, with a virtually exclusive focus on green chemistry (natural phenolic molecules). These include therapeutic strategies for decreasing amyloid- β (Aβ) production, preventing and/or altering A β aggregation, and reducing oligomers cytotoxicity such as curcumin, (-)- epigallocatechin-3-gallate (EGCG), morin, resveratrol, tannic acid, and other natural green molecules. We also examinewhether consideration should be given to potential candidates used outside of medicine and nutrition, through a discussion of two intermediate-sized green molecules, with very similar molecular structures and key properties, which exhibit potential in mitigating Alzheimer’s disease
Thermally-activated charge reversibility of gallium vacancies in GaAs
The dominant charge state for the Ga vacancy in GaAs has been the subject of
a long debate, with experiments proposing 1, 2 or 3 as the best
answer. We revisit this problem using {\it ab initio} calculations to compute
the effects of temperature on the Gibbs free energy of formation, and we find
that the thermal dependence of the Fermi level and of the ionization levels
lead to a reversal of the preferred charge state as the temperature increases.
Calculating the concentrations of gallium vacancies based on these results, we
reproduce two conflicting experimental measurements, showing that these can be
understood from a single set of coherent LDA results when thermal effects are
included.Comment: 4 pages, 4 figure
Understanding long-time vacancy aggregation in iron: a kinetic activation-relaxation technique study
Vacancy diffusion and clustering processes in body-centered-cubic (bcc) Fe
are studied using the kinetic activation-relaxation technique (k-ART), an
off-lattice kinetic Monte Carlo method with on-the-fly catalog building
capabilities. For monovacancies and divacancies, k-ART recovers previously
published results while clustering in a 50-vacancy simulation box agrees with
experimental estimates. Applying k-ART to the study of clustering pathways for
systems containing from one to six vacancies, we find a rich set of diffusion
mechanisms. In particular, we show that the path followed to reach a
hexavacancy cluster influences greatly the associated mean-square displacement.
Aggregation in a 50-vacancy box also shows a notable dispersion in relaxation
time associated with effective barriers varying from 0.84 to 1.1 eV depending
on the exact pathway selected. We isolate the effects of long-range elastic
interactions between defects by comparing to simulations where those effects
are deliberately suppressed. This allows us to demonstrate that in bcc Fe,
suppressing long-range interactions mainly influences kinetics in the first 0.3
ms, slowing down quick energy release cascades seen more frequently in full
simulations, whereas long-term behavior and final state are not significantly
affected.Comment: 11 pages, 12 figures. Updated to post-review manuscrip
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